1. Field of the Invention
The present invention relates to an internal combustion engine including a cylinder head formed with an air intake port having first and second ports including first and second inlet slots opening into a combustion chamber, respectively, wherein a swirl is generated by intake air flowing through the first port.
2. Description of Background Art
An example of an internal combustion engine is disclosed in JP-A-11-50852 in which a swirl is generated in a combustion chamber during an intake stroke in order to improve a combustibility of the lean air-fuel mixture. In this internal combustion engine, a cylinder head is formed with first and second intake ports which communicate respectively with first and second inlet slots opening into the combustion chamber. In addition, first and second intake valves are provided which are driven by a valve mechanism and open and close the first and the second intake ports, respectively. Since the valve mechanism halts the second intake valve and opens and closes only the first intake valve when the internal combustion engine is at a low load, a swirl is generated in the combustion chamber by intake air flowing from the first inlet slot into the combustion chamber. A valve halting mechanism for halting the intake valve is disclosed in JP-A-2000-205038.
In the internal combustion engine disclosed in JP-A-11-50852, a pair of inclined roof surfaces are formed on the bottom of the cylinder head, which defines the upper surface of the combustion chamber. The first and the second inlet slots open on one of the roof surfaces aligned in the direction of the rotational centerline of a camshaft. The axes of valve stems of the first and the second intake valves extend in parallel with each other so as to be orthogonal to the rotational centerline described above. Therefore, the axes of the valve stems of the first and the second intake valves extends in parallel with a second reference plane which includes the axis of the cylinder in a plan view and extends orthogonally to the rotational centerline described above. Therefore, the first inlet slot is included in one of the roof surfaces, wherein the level does not change even when it approaches from the outer peripheral side of the combustion chamber toward the reference plane, in the direction of the rotational centerline. In general, the line of intersection of the plane including the first inlet slot and the plane parallel with a first reference plane which includes the axis of the cylinder and extends in parallel with the rotational centerline described above is in parallel with a third reference plane. Consequently, the length of the line of intersection between the points of intersection with respect to the outline corresponding to the outer periphery of the combustion chamber projecting onto the first plane in the direction of the cylinder axis is smaller than in the case in which the line of intersection does not extend in parallel with the third reference plane. Therefore, there is not enough space for increasing the diameter of the first inlet slot on the combustion chamber having a predetermined diameter. Thus, it is difficult to increase the amount of intake air for generating the swirl by increasing the area of the first inlet slot so as to generate a strong swirl.